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Wabash Formation

Niagaran and Cayugan Series,

Silurian System

Type area and reference sections: The Wabash Formation was named in 1964 for all then so-called "Niagaran" rocks in northern Indiana lying above the Louisville Limestone and exclusive of Silurian rocks then assigned to the Salina Formation (Pinsak and Shaver, 1964, p. 34-47). The type area is in that part of the upper Wabash Valley extending from Carroll County to Huntington County. Five principal reference sections were designated the most complete consisted of rocks cored in the Northern Indiana Public Service Co. Gale M. and Glada Skinner No. 1 well near Royal Center, Cass County (NW¼NW¼ sec. 10, T. 28 N., R. 1 W.). Others consist of some of the classic exposures of the reef-bearing section of northern Indiana. This definition included the preexisting units called the Mississinewa Shale, the Liston Creek Limestone, the New Corydon Limestone (nontypical, of Cumings and Shrock, 1928a, p. 113-117), and the Noblesville Dolomite (obsolete at that time, of Kindle and Breger, 1904, p. 407).

History of name: After considerable post-1964 evidence had accumulated to show that appreciable facies relationship exists between the Wabash and Salina Formations, the Wabash definition was expanded to include the upper part of the Salina Formation as previously defined, and the Salina was given group status to include the Wabash and older rocks in northern Indiana down through the Louisville Limestone and the Limberlost Dolomite (Droste and Shaver, 1982). Before that time, however, Becker (1974), Becker and Droste (1978), Rexroad, Noland, and Pollock (1978), and Droste and Shaver (1980) had defined the southern Indiana occurrence to extend very nearly to the southeastern Silurian outcrop, there being narrowly overlapped in the subsurface by Middle Devonian rocks, and well into the Illinois Basin, there being terminated along a defined vertical cutoff against rocks of the middle and upper parts of the Bainbridge Group. This cutoff leaves an area of about eight-county size unoccupied by Wabash rocks (as defined) in the southwest corner of Indiana.

As noted above, the Wabash in northern Indiana is assigned to the Salina Group, which has a defined southwestern vertical cutoff boundary extending northwestward from eroded edges in central eastern Indiana and, farther on, northward to northwestern Indiana. (See fig. 5 in Droste and Shaver, 1983.) Outside (west and south of) this boundary, the Wabash is not assigned to a group.

Description: Four principal lithologies that intergrade and replace one another spatially characterize the Wabash Formation: (1) calcareous silty dolomite and dolomitic silty limestone that are gray, dense to fine grained, and massive (when fresh) and that are characteristic of, but not confined to, the Mississinewa Shale Member in the lower part of the formation; (2) limestone, dolomitic limestone, and dolomite that are light colored, mostly finely granular, cherty but otherwise fairly pure, and slabby bedded (in weathered exposures) and that are characteristic of, but not confined to, the upper part of the formation, especially the Liston Creek Limestone Member and less so the Kenneth Limestone Member; (3) dolomite and dolomitic limestone that are generally in shades of light tan to dark brown (but including grayish and greenish colors), are generally micritic to fine grained, are generally nonfossiliferous (usual sense), and become strikingly color banded and thinly laminated over broad areas and that are characteristic of the Kokomo Limestone Member and especially of the upper Wabash rocks (unnamed to member) in the northern two tiers of Indiana counties some greenish shale is included in the latter lithology in those northern counties; and (4) light-colored granular massive vuggy, nearly pure dolomite and limestone and bluish-gray carbonate mudstone that are widely distributed in bank, reef, reef-detrital, and biohermal facies throughout much of the formation.

The latter lithology has been widely referred to as the Huntington Lithofacies (not a rock-stratigraphic term and not confined to the Wabash) as first recommended by Pinsak and Shaver (1964, p. 39-40), partly as a salvage expedient for the classic but misused term Huntington Dolomite (Limestone, Stone). The reef facies within the Wabash exists in probably thousands of modest-sized to very large so-called patch reefs and pinnacle reefs and in barrierlike, but yet poorly understood, features called the Fort Wayne Bank (marginal to the Michigan Basin) and the Terre Haute Bank (marginal to the Illinois Basin). (See descriptions, including actual depictions, of such features in Cumings and Shrock, 1928a, 1928b; Shaver and others, 1978; Droste and Shaver, 1980, 1982.) At least part of such features are made up of the upper, coalesced parts of reefs that are discrete in their lower parts. The reef facies in southwestern Indiana and adjacent Illinois includes the thickest Silurian reef deposits known in the United States. Some are nearly 1,000 feet thick, but part of that thickness is in the pre-Wabash stratigraphic interval (Lowenstam, 1949; Becker and Droste, 1978; Bristol, 1974; Droste and Shaver, 1980; and Droste and Shaver, in preparation).

The Wabash Formation is underlain conformably by the Louisville Limestone. The boundary is placed at the bottom of a thick transitional interval (Rexroad, Noland, and Pollock, 1978, p. 2), or it is underlain in similar manner by the Pleasant Mills Formation. The difference is only a defined difference in names. (See the Pleasant Mills [also Salina] limit line in fig. 6 of Droste and Shaver, 1982). The Wabash, where present, underlies nearly everywhere the Muscatatuck Group (mostly Middle Devonian) unconformably. Possibly in farthest northeastern Indiana the Wabash underlies even older Lower Devonian rocks, although such rocks are presently included in the Muscatatuck Group. Some Wabash rocks, including reef rocks, along the defined Wabash cutoff in far southwestern Indiana underlie the New Harmony Group (Lower Devonian), possibly both conformably and unconformably.

The Wabash Formation ranges in thickness from zero along its southeastern eroded edge to a regional (nonreef) thickness of more than 400 feet (122 m) in southwestern Indiana, to about 250 feet (76 m) in central western Indiana, to more than 400 feet (122 m) in the Newton County area (northwestern Indiana), and to 200 feet (61 m) in northeasternmost Indiana. (See Becker, 1974, p. 21, and Droste and Shaver, 1982, fig. 7, and in preparation.)

Correlation: The hundreds of classically studied macrofossils from Wabash reef and interreef rocks have been assigned stratigraphically secure positions (Shaver, 1974b). Among those to which greatest biostratigraphic importance may be attached are the pentamerid brachiopods Kirkidium cf. K. knighti (low in the formation) and K. cf. K. laqueatum (high in the formation), the graptolite Monograptus falciformis (= M. bohemicus; low in the formation), and the mollusk Megalomus canadensis (known high in the formation but probably has a lower range to below the formation).

Microfossil studies have been undertaken more recently and have recorded conodont taxa that are considered to be elements of zones ranging upward from within the Zone of Kockelella to within the Zone of Ozarkodina eosteinhornensis. The acritarch zone characterized by Deunffia eisenacki has been identified in lower Wabash rocks. Ostracod species identified as Dizygopleura hallii and Thlipsurella parva and associated species are found very high in the formation.

These fossils collectively indicate an age ranging from within Ludovian time into Pridolian (late Niagaran to possibly very late Cayugan). (See the separate article for constituent units and summaries in Shaver and others, 1971, p. 53-54, and 1985; Droste and Shaver, 1977, p. 99-100; and Shaver and Sunderman, 1983, p. 148.)

Physical tracing, sequential stratigraphy, and geophysical logging yield other correlative evidence, so that the evidence of all kinds supports these approximate to close correlations: deeper part of the Illinois Basin in Indiana, Kentucky, and Illinois, much of the Moccasin Springs Formation through the Bailey Limestone; northern Illinois, middle part of the Racine Formation through all the Racine that is uneroded; Michigan Basin, from either the B salt (unit) or C unit of the Salina Group to as high as the G unit and possibly even as high as the overlying Bass Islands Group; south-central Kentucky and adjacent Tennessee, the Dixon Formation through the Decatur Limestone; and New York, from within the Vernon Shale (Salina Group) probably to Silurian formations above the Salina Group.

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